Ocean darkening
Understanding the scale and impact of darker oceans
Is the ocean getting darker?
More than one fifth of the global ocean has been affected by ocean darkening over the past two decades and accounts for the one of largest losses of global habitat quantified in the world. This darkening reduces the amount of ocean available for animals that rely on the sun and moon for their survival.
We rely on the ocean for the air we breathe, the fish we eat and our ability to fight climate change – and the general health and wellbeing of the planet. Ocean darkening could affect huge numbers of the planets marine species and ecosystem services provided by the ocean as a whole.
Focus on the photic zone
The top 200m of the ocean – the photic zone – is one of the most productive habitats on the planet and fundamental to the maintenance of healthy global biogeochemical cycles – the natural pathways that essential elements of living matter are circulated. Home to 90% of all marine life and critical for global nutrient and carbon budgets and sustaining global fish stocks, the photic zone is where sunlight and moonlight drive these ecological interactions.
If the photic zone is reduced by 50m in large areas of the ocean, marine life that need light will be forced closer to the surface, leading to more competition for food and other resources. This could fundamentally change the marine ecosystem.
Why is the ocean getting darker?
Ocean darkening occurs when changes in the optical properties of the oceans reduce the depth to which sufficient light penetrates to facilitate biological processes guided by sunlight and moonlight.
With more evidence required, ocean darkening is likely the result of a combination of nutrient, organic material and sediment loading near the coasts from agricultural runoff an increased rainfall. In the open ocean, it is likely down to changes in algal bloom dynamics and shift in sea temperatures, which both reduce light penetration into surface waters.
Ocean darkening drivers
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aIncreased phytoplankton blooms
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aIncreased rainfall
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aLand use change such as deforestation, agricultural intensification and urbanisation
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aDeglaciation – the retreat or melting of ice sheets and glaciers
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aIncreased temperatures
Changing lightscapes
The surface oceans have changed colour over the last 20 years, indicating global shifts in the ocean ecology. Overall, this indicates the oceans are getting darker and these effects are not just restricted to coastal regions, but affecting large swathes of the open ocean.
However, the picture is not solely of a darkening ocean, with around 10% of the ocean – more than 37 million km2 – becoming lighter over the past 20 years.
Overall, these changing lightscapes are affecting our oceans and the marine life within it.
Documenting the impact – a global outlook on ocean darkening
Coastal darkening is well documented from analysis of numerous long-term sensing products and in situ datasets that span several decades. However, long-term datasets across large spatial scales are needed to account for climate and variability in the ocean, seasonal and regional variability, and determine the effects of large scale processes on land.
Ecologist
Dr Thomas Davies
, Associate Professor of Marine Conservation at the ̽»¨ÊÓƵ, and Professor Tim Smyth at ̽»¨ÊÓƵ Marine Laboratory have collaborated on marine light pollution research for over 10 years. Together, they are advancing the scientific boundaries of coastal darkening research, evolving toward a globally contextualised understanding of ocean darkening. Their collaboration has evidenced the scale of ocean darkening, and they are now looking at its drivers and ecological impacts.
Discoveries include:
- 21% of the global ocean has darkened over the last two decades, an area nearly 10 times greater than the total area of deforested land
- More than 9% of the ocean – an area of more than 32 million km2, similar in size to the continent of Africa – has seen photic zone depths reducing by more than 50m, while 2.6% saw the photic zone reduced by more than 100m.
- Ocean darkening is not restricted to coastal regions, but affecting large swathes of the open ocean, widespread in the major oceans particularly in polar regions, the North East Atlantic and North West Pacific Oceans.
The implications of ocean darkening for marine ecology and the ecosystem services provided by the surface oceans are currently unknown but likely to be severe.
Darkening of the global ocean
Writing in Global Change Biology,
Dr Thomas Davies
and Professor Tim Smyth share their findings that 21% of the global ocean experienced a reduction in the depth of its photic zones, home to 90% of all marine life.
They used a combination of satellite data and numerical modelling to analyse annual changes in the depth of photic zones all over the planet. Using data from NASA’s Ocean Colour Web, which breaks the global ocean down into a series of 9km pixels, the data enabled them to observe changes on the ocean surface for each of these pixels, while an algorithm developed to measure light in sea water was used to define the depth of the photic zone in each location.
The full study – Davies and Smyth: Darkening of the Global Ocean – is published in Global Change Biology, DOI: .
A map focused on the UK and North Atlantic Ocean showing changes identified – red areas indicate regions where the oceans are getting darker, while blues indicate regions where they are getting lighter.
Marine light pollution
Understanding and managing the impact of artificial light at night (ALAN) for healthy seas
Dr Thomas Davies has been driving the strategy for marine light pollution research and mobilising policy action since he published the first paper on the topic in 2014. More recently, working in partnership with Professor Tim Smyth at ̽»¨ÊÓƵ Marine Laboratory, this research is now combining laboratory and field experiments with remote sensing to deliver insights into how changing lightscapes impact marine organisms and ecosystems.
